Oftentimes it will be advantageous that a certain surface, due to its fragility and/or environmental sensitivity, be secured with an appropriate protective coating or layer. In this regard, the application of a protective overcoat onto such a surface by means of lamination has become a favored practice. One surface that benefits from such lamination is an image surface of a thermal imaging medium, such as described in International Patent Application No. PCT/US87/03249 (Publication No. WO 88/04237) (Etzel), the image surface being formed after imagewise development of the medium's image-forming layer.
More particularly, International Patent Application No. PCT/US87/03249 describes a thermal imaging medium and a process for forming an image in which a layer of a porous or particulate imaging material (preferably, a layer of carbon black) is deposited on a heat-activatable image-forming surface of a first sheet-like element, the layer having a cohesive strength greater than its adhesive strength to the first sheet-like element. Portions of this thermal imaging medium are then exposed to brief and intense radiation (for example, laser scanning), to firmly attach exposed portions of the imaging material to the first sheet-like element. Finally, those portions of the imaging material not exposed to the radiation (and thus not firmly attached to the first sheet-like element) are removed, thereby forming a binary image surface comprising a plurality of first areas where the imaging material is adhered to the first sheet-like element and a plurality of second areas where the first sheet-like element is free from the imaging material.
In an embodiment of the thermal imaging medium described by International Patent Application No. PCT/US87/03249, the imaging material is covered with a second laminated sheet-like element so that the imaging material is confined between the first element and this second element. After imaging and separation of the second element (with the unexposed portions of the imaging material) from the first element, a pair of image surfaces is obtained. The first image surface comprises exposed portions of image-forming substance more firmly attached to the first element by heat activation of the heat-activatable image-forming surface. The second image surface comprises non-exposed portions of the image-forming substance carried or transferred to the second sheet element. Either of the pair of image surfaces may, for reasons of informational content, aesthetic or otherwise, be desirably considered the principal image surface, and all of the following discussion is applicable to both types of image surface.
While the image-forming process described in International Patent Application No. PCT/US87/03249 is capable of producing high quality, high resolution images, the durability of the image surfaces produced by this process may be inappropriate for certain desired applications. In the finished image surface, the porous or particulate imaging material, typically carbon black admixed with a binder, lies exposed (unprotected). The image may, thus, be vulnerable to being smeared, damaged or removed by, for example, fingers or skin surfaces (especially if moist), solvents or friction during manual or other handling of the image.
In consideration of its unprotected condition it may be desirable to protect the image surface of the developed thermal imaging medium by the application of a protective overcoat, such as a thin, transparent, but durable radiation (for example, laser scanning), to firmly attach exposed portions of the imaging material to the first sheet-like element. Finally, those portions of the imaging material not exposed to the radiation (and thus not firmly attached to the first sheet-like element) are removed, thereby forming a binary image surface comprising a plurality of first areas where the imaging material is adhered to the first sheet-like element and a plurality of second areas where the first sheet-like element is free from the imaging material.
In an embodiment of the thermal imaging medium described by International Patent Application No. PCT/US87/03249, the imaging material is covered with a second laminated sheet-like element so that the imaging material is confined between the first element and this second element. After imaging and separation of the second element (with the unexposed portions of the imaging material) from the first element, a pair of image surfaces is obtained. The first image surface comprises exposed portions of image-forming substance more firmly attached to the first element by heat activation of the heat-activatable image-forming surface. The second image surface comprises non-exposed portions of the image-forming substance carried or transferred to the second sheet element. Either of the pair of image surfaces may, for reasons of informational content, aesthetic or otherwise, be desirably considered the principal image surface, and all of the following discussion is applicable to both types of image surface.
While the image-forming process described in International Patent Application No. PCT/US87/03249 is capable of producing high quality, high resolution images, the durability of the image surfaces produced by this process may be inappropriate for certain desired applications. In the finished image surface, the porous or particulate imaging material, typically carbon black admixed with a binder, lies exposed (unprotected). The image may, thus, be vulnerable to being smeared, damaged or removed by, for example, fingers or skin surfaces (especially if moist), solvents or friction during manual or other handling of the image.
In consideration of its unprotected condition it may be desirable to protect the image surface of the developed thermal imaging medium by the application of a protective overcoat, such as a thin, transparent, but durable layer, such as described in International Patent Application No. PCT/US91/08345 (Publication No. WO 92/09930) (Fehervari et al.); and allowed U.S. Patent Application Ser. No. 08/065345 (Bloom et al.).
Lamination of protective overcoats, such as those described in the cited patent applications, has been accomplished using a continuous roll or carrier web to conduct the durable layer to the imaged sheets, the durable layer typically being associated with an adhesive layer. Fed through a nip existing between paired compression rollers, activation energy to fuse the durable layer to the imaged sheet is provided by thermal heating elements integrated into or with the paired compression rollers. Generally, the top roller is actively heated and the bottom roller allowed to reach a steady state temperature well above room temperature by conduction. Lamination is effected by the cooperative influences of both compression and thermal heating. While such method has provided good results, aspects intrinsic to such lamination techniques may become inconsistent for certain applications or when certain functionalities are desired. Some of such aspects may be noted.
First, it will be appreciated that heated compression rollers generally have considerable thermal mass. When energized (heated) from a cold start, a delay may be anticipated before such rollers reach a surface temperature adequate to conduct satisfactory lamination.
Further, since heat is transferred by conduction from a heated roller to the receiving surface of a web material, matter between the heated roller and the receiving surface is heated to at least the temperature attained at the receiving surface. This imposes substantial temperature restrictions when using a low Tg web material, such as polyester. Such material's thermoplastic nature compels comparatively precise control of post-nip material geometry to prevent undesirable web distortion.
Further, heat recovery rate characteristics of the roller used for heat transfer to the laminate may limit the rate at which lamination can be conducted if the web length passing through the nip is longer than the circumference of the heated roller. If full equilibration is not achieved, then conditions will change abruptly as the energy depleted portion of the roll surface begins the next cycle.
Further, for systems with large surface areas, heat loss by convection and radiation can become substantial. Heat loss requires provision of costly "make up" energy. Lost heat can also affect materials or electric components within the vicinity of the laminator over time.
Further, large physical structures once brought to thermal equilibrium present a problem if service to elements in their vicinity is required. Safety may be compromised when working on or near these heated components. The time required to cool to a safe level once power is cut can be substantial.
In light of the above, need exists for a laminating system useful for laminating a protective overcoat onto a receiving surface yet minimizing or obviating difficulties that manifest in certain lamination processes that utilize heated compression rollers.